This invention relates generally to variable speed rotating electrical machinery, and more particularly to variable speed induction motor drive systems operating at a leading power factor.
An example of an induction motor drive system of this particular type is provided in a copending application, Ser. No. 331,108, entitled "Leading Power Factor Induction Motor Drive", filed Dec. 15, 1981 in the names of Gabor Kalman et al. The application describes an induction motor drive system including a main machine and a smaller auxiliary machine, installed in a common housing. The rotors of the two machines are mounted on a common shaft, and both rotors have a squirrel-cage construction. The rotor bars of the main machine and of the auxiliary machine are concatenated at their adjoining ends and are connected to conventional end rings at their extremities. The main machine is supplied from a line-commutated inverter, and the auxiliary machine is supplied from a separate, forced-commutated inverter. The two inverters are supplied by separate variable dc link voltage sources, such as controlled rectifiers or chopper-type input inverters.
AC induction motors typically operate at a lagging power factor, and are therefore inherently incompatible with line-commutated inverters, which function properly only if they supply a leading power factor load. A static inverter that is capable of supplying lagging power factor loads must have auxiliary circuitry to force the communication of its solid state switching devices. This auxiliary circuitry makes such a forced-commutated inverter significantly more expensive than a line-commutated inverter of comparable power rating.
In the induction motor drive system described in the aforementioned copending application, the forced-commutated inverter for the auxiliary machine is specially configured such that the stator of the main machine appears to have a leading power factor. In particular, the frequency and voltage levels of the ac signals supplied by the forced-commutated inverter to the auxiliary stator are coupled through the respective auxiliary and main rotors to the main stator, causing its current to lead its voltage. This permits the use of a line-commutated inverter to drive the main machine, thereby providing a significant cost savings for a machine of a given size.
The induction motor drive system described in the aforementioned copending application produces a high starting torque and is suitable for use in applications requiring power ratings of up to several hundred horsepower. Also, its use of a line-commutated inverter in place of a forced-commutated inverter provides a significant cost savings over known prior systems. However, that is a need for even further cost savings in some situtations, such as where the system's starting torque need not be particularly high. The present invention fulfills this need.